linux-kernel - Re: uprobes && shmem (Was: uprobes: Shift ->readpage check from __copy_insn() to uprobe

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Date:	Fri, 16 May 2014 21:29:49 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Hugh Dickins <hughd@...gle.com>
Cc:	Johannes Weiner <hannes@...xchg.org>, Ingo Molnar <mingo@...e.hu>,
	Denys Vlasenko <dvlasenk@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
	linux-kernel@...r.kernel.org
Subject: Re: uprobes && shmem (Was: uprobes: Shift ->readpage check from
	__copy_insn() to uprobe_register())

On 05/16, Hugh Dickins wrote:
>
> On Fri, 16 May 2014, Oleg Nesterov wrote:
> > On 05/16, Oleg Nesterov wrote:
> > >
> > > copy_insn() fails with -EIO if ->readpage == NULL
> >
> > In particular, this means that we can not probe the binaries on tmpfs.
> > This is pity.
>
> Yes, that is a pity: thanks for noticing.

Thanks to Denys ;)

> > It seems that the potential fix is trivial, copy_insn() could use
> > shmem_getpage_gfp(). But, is there any way to figure out that this
>
> shmem_getpage_gfp() itself is static: please use
> shmem_read_mapping_page(mapping, pgoff): inline in linux/shmem_fs.h,
> calls shmem_read_mapping_page_gfp() in mm/shmem.c (a very few places
> need to override gfp_mask too: you do not), calls shmem_getpage_gfp().

Even better, thanks,

> > inode/mapping/aops/whatever is actually shmem?
> >
> > I am looking at shmem_get_inode() and I see nothing which could help,
> > and shmem_aops/etc are all static.
>
> On 3.15 and later, you're in luck: Hannes added bool shmem_mapping(mapping)
> in his 0cd6144aadd2 "mm + fs: prepare for non-page entries in page cache
> radix trees"; and I just checked, it builds for "tiny" !CONFIG_SHMEM too.

Heh. I need to do git pull more often, I guess. Great.

> If you're backporting to an earlier kernel, it would probably be best
> to add in a very small patch, extracting just shmem_mapping() and its
> linux/mm.h declarations from 0cd6144aadd2.
>
> I notice shmem_mapping() checks backing_dev_info,
> whereas shmem_get_mapping_page_gfp() checks a_ops: no problem in that.
> But it reminds me that you should test uprobe on SysV SHM when you're
> done: again I think no problem, but there's an incestuous relationship
> between shm and shmem that can catch us out when adding such checks.

Hugh, thanks a lot!

Oleg.

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